Field of the Invention
The present invention relates to a projection zoom lens for use in a projection optical system of a projector, in particular, to a zoom lens suitable for enlarging an original image formed by light beams modulated by a micro device to be projected on a screen, and a projector equipped with the zoom lens.
Description of the Related Art
A front projection type projector which projects an image on a projected surface (screen) placed in front of the projector is widely used in, for example, enterprises for presentation, schools, or homes. Many light modulation elements (light valves) for use in a projector are transmission type or reflection type liquid crystal elements. However, a micro device as typified by a digital micromirror device (DMD), which is advantageous for downsizing and improving brightness, is often used.
The DMD is an element which generates a projection image with light reflected by a micromirror. The turning angle of the micromirror is about ±10°, and effective reflection light and ineffective reflection light can be switched by the angle differences. Effective incident control of the effective reflection light and the ineffective reflection light from the DMD is required for a projection lens of a projector using the DMD. For this reason, it is desirable to arrange a projection lens in the normal direction on the side of the DMD. In order to achieve such an arrangement, an illumination light source and the projection light source are placed in approximately the same position. The arrangement of the projection lens in the normal direction on the side of the DMD is limited as described above.
In view of the above-described limitation, it is necessary for the projection lens of the projector using the DMD to be downsized such that the lens diameter on the side of the light valve does not interfere with the illumination light system, and to have long backfocus. Therefore, a relatively low-power zoom lens and a telephoto zoom lens are suitable for the projection lens of the projector using the DMD.
In recent years, a lens having both high-power zoom and wide-angle zoom is required for such a projection lens, and a projection lens corresponding to such a lens is known (refer to JP 2011-69959A, JP 2011-69957A, JP 2003-015038A, JP2005-292260A).
The lens described in JP 2011-69959A and JP 2011-69957A is a projection zoom lens aimed at high power in the fifth and sixth groups, and having a zoom ratio of about×1.5. Such a zoom lens has a half-field angle ω of 30° although the aberration is well corrected. The lens described in JP 2003-015038A and JP2005-292260A is a wide-angle zoom lens of a projection optical system, but it is an LCD type telecentric optical system, which cannot be adopted for the optical system using the DMD. Accordingly, it is difficult for the known projection lens of the projector using the DMD to achieve both high power and a wide angle of view.